WMI means Windows Management Instrumentation. It's a way / api and specification, well, to manage Microsoft Windows systems based on COM and DCOM-Interfaces. For a deeper explanation, take a look at http://de.wikipedia.org/wiki/Windows_Management_Instrumentations .
See Matthias Hoffmann - Tcl-Code-Snippets - tcom & wmi - Examples for WMI examples using tcom.
See TWAPI and WMI for WMI examples using TWAPI.